1. Field of the Invention
This invention relates generally to a color image reproducing apparatus, and more particularly, to a color image projecting apparatus using three cathode ray tubes as image sources.
2. Description of the Prior Art
It is known in the art to use three cathode ray tubes each of which produces a different primary color image to be projected on a screen as a color image.
A prior art color projector has been proposed in which monochrome cathode ray tubes, provided with red, green and blue primary color phosphors, are driven with red, green and blue color signals to produce three respective primary color images. The three primary color images are projected through a projecting lens apparatus which converges them on a screen to reproduce a color image thereon in enlarged scale. Since the incident angles of three the light images at the screen are so different, the use of a high gain (high directivity) screen results in a color shading depending upon viewing position. Accordingly, it is necessary to use a low gain (low directively) screen so that sufficient luminance or brightness cannot be obtained. Furthermore, in the prior art color projector, due to the fact that the optical paths from the respective cathode ray tubes to the screen are different and their projecting angles on the screen also are different, keystone distortion must be corrected to obtain registration over the screen. In addition, when the size of a picture on the screen is changed, it is necessary to correct the keystone distortion, the position of the picture, the focal position of the projecting lens apparatus, the optical axes thereof and the optical axes of the cathode ray tubes, which is troublesome and requires complicated optical apparatus, electrical circuitry and difficult adjustments. Moreover, the prior art color projector requires a three lens system, which is expensive.
It is also known in the art to use a pair of crossed dichroic mirrors disposed in front of three cathode ray tubes for synthesizing a three color image therefrom and projecting the synthesized image on a screen through a single lens. In this case, the light axes of three cathode ray tubes coincide with each other; however, owing to light loss through and on the dichroic mirrors, the color image projected on the screen cannot have sufficient brightness.
Furthermore, since the viewing angles of the cathode ray tubes to the dichroic mirrors are rather large, the permeable or reflection characteristics of the dichroic mirrors are much different at the centers and at the ends of the dichroic mirrors and accordingly considerable color shading appears.